It makes sense, and sounds natural. However, I would change "could" to "would." "Could" means "possibly" whereas "would" indicates "definitely." Also, you didn't want her to just look at you (see), you wanted her to know (develop a relationship with) you. "If you had given me a chance, I would have been the best man you had ever known in your life."

For written English it is quite good. I would use "known" instead of "seen" and begin with "if you would have" For spoken English you would most likely use "double contractions" so you would say/hear "If you'd've (you would have) given me a chance, I could've been the best man you had ever known." Although I mist admit that it sounds more like a song lyric than anything you would likely hear. Hope this helps, J

The sentence sounds natural, as long as you say known. Other ways that you can say this are: 1) If had just given me a chance, I could have been the best man you ever met. 2) If you had just given me a chance, I could’ve been the best thing that ever happened to you.

Thank you Neal.

Mehrdad, it makes perfect sense. The only change I'd suggest is you might say: "If you had given me a chance,I could have been the best man you had ever had in your life." It is subtly different but the same meaning.